This invention relates to a driving circuit for driving a stepping motor, and more particularly to a stepping motor driving circuit which has a failsafe function for protecting the circuit in the event of an electrical fault in the circuit.
Conventionally, a stepping motor driving circuit is generally provided with a protective device which is operable to protect the circuit in the event of a short circuit between a circuit element and wiring or the like, by preventing the stepping motor and/or circuit elements from being damaged or broken by excessive current (overcurrent) flowing in the motor and/or the circuit elements.
The conventional protective device includes the following types:
i) a type employing a fuse which is cut by overcurrent;
ii) a type employing a resistance having a low resistance value and arranged in the driving circuit at a suitable place for detecting overcurrent such that a relay or like means is actuated upon desection of overcurrent by the resistance to cut off the supply of driving current to the stepping motor; and
iii) a type having means for applying driving pulses to the stepping motor and detecting the rotational angle of the motor driven by the pulses, means for detecting a fault in the driving circuit based upon the detected rotational angle and the number of driving pulses applied to the motor, and means for cutting off the supply of driving current to the motor upon detection of a fault.
The above type i) suffers from low responsiveness, and the type ii) has a drawback of large heat loss due to the use of a resistance. On the other hand, the type iii) is not capable of selectively detecting only electrical faults, since it can erroneously judge that a fault has occurred, even when the means for detecting the rotational angle becomes faulty, when the motor skips, or when there takes place a mechanical fault.
Further, all the above-mentioned types are adapted to detect a fault only after excessive current has flown due to occurrence of the fault. Therefore, the motor and its related parts can be broken or damaged before a fault is detected.